The present invention relates to a process for reducing the adsorption onto rock material of polymers present in fluids used in drilling, completion and fracturing operations of oil and/or gas wells.
Water-based drilling, fracturing and completion fluids (in particular fluids for gravel packing) used in the treatment of reservoirs formations, often contain hydrosoluble polymers.
These polymers may be either synthetic (for example polyacrylamides) or natural; in this case the polymers are known as biopolymers. Typical examples of the latter (mostly of the polysaccharide or cellulose type) are scleroglucan, xanthan gum, guar gum, hydroxypropyl guars, welan gum, galactomannans and hydrophobically modified galactomannans, succinoglucan, celluloses variously modified with hydrophobic chains, hydroxyethyl celluloses or their mixtures.
These polymers are used to obtain the viscosities required in relation to the performances desired. For example, in the case of drilling fluids, the presence of these polymers improves the suspending and transport properties of the fluid; as a secondary effect, the filtrating properties of the fluid are also improved, thus reducing the quantity of filtrate which can invade the reservoir.
During the optimizization of the various fluids used in the drilling, fracturing and completion of oil and/or gas wells, it has been demonstrated however that these polymers have the disadvantage of being strongly adsorbed on rock formations.
This adsorption probably causes a reduction in the well productivity, as it modifies the permeability of the reservoirs, seriously damaging them.
In fact, when a biopolymer solution (for example a scleroglucan) is injected into a porous medium, for example sandstone core of the Clashach type, mainly consisting of quartz (98%) and a small percentage of clays (2%), the polymer is adsorbed onto the rock and remains almost totally adsorbed even after displacement with an aqueous solution of KCl 3%. The effect of this adsorption can be seen comparing the permeability of the porous medium before the polymer injection and after its displacement with the aqueous solution of KCl 3%. A comparison between the initial permeability to water of the porous medium (ki) and the final permeability (kf), i.e. after displacement of the polymer with a solution of KCl 3%, shows a drastic decrease in the permeability to water due to the adsorbed polymer.
A process has now been found for at least partly reducing the drawbacks relating to the adsorption of polymers onto rock matrix.
In accordance with this, the present invention relates to a process for reducing the adsorption of hydrosoluble polymers onto rock matrix, which comprises putting the rock matrix in contact with an aqueous solution of polymers comprising sacrificial agents, preferably selected from ionic and non-ionic oligomeric surface-active agents.
The hydrosoluble polymers are selected from biopolymers and synthetic polymers. Typical examples of these polymers are indicated above.
The term xe2x80x9coligomeric surface-active agentsxe2x80x9d refers to surface-active agents having a molecular weight ranging from 150 to 5000, preferably from 200 to 2000. They are selected from cationic, anionic, amphoteric, non-ionic surface-active agents.
In the preferred embodiment, the oligomeric surface-active agents are of a non-ionic nature with an HLB ranging from 11 to 16, and their concentration varies from 0.5 g/l to 20.0 g/l, preferably from 2.0 to 10.0 g/l.
Typical examples of non-ionic surface-active agents which can be used in the process of the present invention are those belonging to the group of alkylpolyglucosides. The latter are selected from those having general formula (I)
Rxe2x80x94Oxe2x80x94(Axe2x80x94O)x-(G)y-(D)zxe2x80x83xe2x80x83(I)
wherein:
R is a non-substituted or hydroxy-substituted, linear or branched C8-C26 alkyl group;
A is a linear or branched C2-C4 alkylene group, or a bifunctional residue of a polyalcohol after the removal of 2 carboxyl groups on 2 carbon atoms;
G is a saccharide residue which remains after the removal of z hydrogen atoms from all the non-glucosidic hydroxyl groups and the removal of the glucosidic hydroxyl group of a hexose/pentose reducing sugar bound to Axe2x80x94O or Rxe2x80x94O (wherein x=0) by means of an O-glycosidic bond;
D is an acyl residue of an organic acid, bound to an oxygen atom of the residue (G)y having the formula: xe2x80x94C(xe2x95x90O)M wherein M is a linear or branched C1-C18 alkyl or alkenyl group, non-substituted or substituted with xe2x80x94OH;
x=0-10 is the condensation degree of A;
y=0-10 is the average esterification or etherification degree (G)y.
Typical examples of alkylpolyglucosides having general formula (I) are:
APG(copyright) 325 Glycoside, alkylpolysaccharide ether with HLB=13.0, MW=420 and alkyl=C9/11 (Henkel);
APG(copyright) 600 Glycoside, alkylpolysaccharide ether with HLB=11.5, MW=420 and alkyl=C12/14 (Henkel);
APG(copyright) 625 Glycoside, alkylpolysaccharide ether with HLB=12.0, MW=455 and alkyl=C12/14 (Henkel);
Glucopon(copyright) 225; HLB=13.6; alkyl C8/12 (Henkel);
Glucopon(copyright) 600; HLB=11.6; alkyl C12/16 (Henkel);
Glucopon(copyright) 625; HLB=12.1; alkyl C12/16 (Henkel);
The process of the present invention comprises injection of the fluid prepared according to the following procedure: the hydrosoluble polymer is dissolved in H2O or aqueous solutions of NaCl, KCl, CaCl2, using a stirrer (for example of the Silverson type), the surface-active agent is then added, which, as a result of its physico-chemical characteristics, also favours the dispersion of possible other fluid additives, such as starches and celluloses used as filtrate reducers. The fluid thus prepared is used in the well, according to specific operative procedures for the various drilling, fracturing and completion fluids.
The process of the present invention is most widely applied in the oil upstream field, with particular reference to all drilling and completion operations of oil and/or gas wells involving the use of polymers, and where the prevention of the formation damage is of fundamental importance for avoiding reduction in the well productivity. The process of the present invention can therefore help to prevent formation damage during the drilling (drilling fluids), fracturing (fracturing fluids), completion (fluids for gravel packing, for example) phases.
The process of the present invention can also be applied for reducing the adsorption of hydrosoluble polymers onto any inorganic material, not only onto rock matrixes.
Furthermore, in another embodiment, the process of the present invention can be carried out in two steps: in the first phase, an aqueous solution of sacrificial agent, with a concentration of 2 to 100 g/l, preferably from 5 to 20 g/l, is injected into the rock matrix; in the second phase, the fluid containing the polymer is injected into the well adopting the procedures of the different operations (drilling, fracturing, completion).